1. Field of the Invention
The present invention relates to an image forming apparatus including a feeding device for feeding a recording material by blowing air to the recording material, and a transfer device for electrostatically transferring a toner image onto the recording material.
2. Description of the Related Art
Conventional image forming apparatus such as copying machine, printer and the like includes a sheet feeding device for sequentially feeding the sheets stacked on a sheet stacking portion one by one starting from the sheet at the top by means of a pickup roller, and thereafter, separating the sheets one by one by means of a separating portion and feeding the sheet to an image forming portion.
Cut sheets are used when successively feeding the sheets in such sheet feeding device, but such cut sheets are normally limited to quality paper or plain paper recommended by the copying machine manufacturing company. Various separation methods have been conventionally used to reliably separate and feed the sheets one at a time. The various methods include a separation pad method of contacting a friction member to the feed roller at a predetermined pressure to prevent feeding in overlapped manner.
In another separating method, or the retard separating method, the separating portion is configured by a feed roller that rotates in a sheet conveying direction, and a separation roller that is driven in a direction opposite the sheet conveying direction at a predetermined torque and that contacts the feed roller at a predetermined pressure. In the retard separating method, only the top sheet of the sheet stack sent out by the pick up roller is passed, and the other sheet fed along with the top sheet is returned to the sheet stacking unit side by the separating portion to prevent feeding in overlapped manner.
The sheets are reliably separated one by one by optimizing the return torque and the pressurizing force of the separation roller in consideration of the frictional force of the sheets to be fed to reliably separate and feed the sheets in such separating methods, for example, in the retard separating method.
Recently, request to form images on the sheets such as coated paper, which surface is performed with coating process to give whiteness and glaze from demands of the market of colorization, is increasing in addition to super thick paper, OHP sheet, art film with diversification of the sheets (recording material, recording medium).
However, when feeding super thick papers, the weight of the super thick paper acts as resistance in conveying, and the sheets get jammed as they cannot be picked up. The surface of the sheets made of resin material that are easily charged such as OHP sheet and art film is gradually charged when the sheets are rubbed against each other in the feeding operation under low humidity environment. Since the sheets attach to each other by Coulomb force, the sheets may not be picked up or may be fed in overlapped manner.
The coated sheets with coating material including paint applied to the surface of the paper have a property of attaching to each other particularly when stacked in an environment of high humidity, and thus may not be picked up or may be frequently fed in overlapped manner.
This is because in cases of special sheets such as the above, the frictional force itself of the sheets is equal to or less than plain paper, but the absorption force is high. That is, the sheets are absorbed to each other at a force much higher than the frictional force of the sheets by the absorption force due to the friction charge under low humidity environment in the case of resin material sheet, or by absorption force under high humidity environment in the case of coated sheets, and thus the sheets may not be adequately separated with the conventional separating method.
That is, since only the frictional force between the sheets is considered in the conventional separating method, the sheets cannot be reliably separated if absorption force other than the frictional force is acting.
The separating and feeding method using air separation is adopted in printing industry and some copying machines to release the very high absorption force between the sheets. This is a method of separating the sheets in advance by blowing air from the side face of the sheet stack, picking up the sheets one by one from the top sheet with the absorption between the sheets removed, and separating the sheets one by one at the separating portion arranged at the downstream (Japanese Patent Application Laid-Open No. 11-005643).
The sheets are separated to release the absorption prior to feeding even for sheets having high absorption force in the separating and feeding method equipped with a unit for blowing air from the side face of the sheet stack, as described above, and thus the separation performance enhances compared to the method of using only the friction force as previously described.
The separating and feeding methods in which the air is blown from the side face of the sheet stack includes a method of dehumidifying the sheet by heating the blown air with heater and reducing the absorption force of the coated paper and the like particularly under the high humidity environment (Japanese Patent Application Laid-Open No. 2001-48366).
However, in the feeding device adopting the separating and feeding method of blowing air such as the above, the moisture content of the sheet gradually changes when air is blown. With change in moisture content, the transfer performance with respect to the application bias changes in a secondary transfer portion, and image defect occurs from the middle of the job. In particular, the transfer performance is greatly influenced by the resistance value of the sheet in the electro-photographic method in which the image forming portion transfers the toner image to the sheets using static electricity. Thus, when the resistance value varies in the sheets, transfer becomes uneven, and degradation of image caused therefrom becomes significant, whereby the problem regarding image quality arises.